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ii                                MILK DENTITION                                      5 1

teeth in which the grinding surface is raised into a series of two,
to many, tubercles sharper or blnnter as the case may be ; — sharper
and fewer at the same time in carnivorous and especially in
insectivorous types, more abundant in omnivorous animals. To
this form of tooth the term " bunodont " is applied. There is
no doubt that this is the earliest type of tooth ; but whether the
fewer or the more cusped condition is the primitive one is a
question that is reserved for consideration at the end of the
present chapter. The other type of grinding tooth is known as
" lophodont." This is exemplified by such types as the Perisso-
dactyla and Ungulates generally, and by the Kodents. The tooth
is traversed by ridges which have generally a transverse direction
to the long axis of the jaw in which the tooth lies. The ridges

f.a

I                         *                     »        ***     "*"•   P

2flG. 36.—Molar teeth, of Aceratherium,yHatycepha,luinf x ^ m.l-m.3, Molars ; »iA, meta-
lopli; ^?.l-jp.4» premolars; ph, protoloph; psj, parastyle fossa ; te, tetartocone.
(After Osborn.)

may be regarded as having been developed between tubercles
which they connect and whose distinctness as tubercles is
thereby destroyed. Lophodont teeth are only found in vegetable-
feeding animals.
The special characteristics of the teeth of various groups of
animals will be considered further under the accounts of the
several orders of recent and fossil Mammalia.
A very general feature of the teeth of the Mammalia is what
is usually termed the diphyodont dentition. In the majority of
cases there are two sets of teeth developed, of which the first
lasts for a comparatively short time, and is termed on account of
its usual time of appearance the "milk dentition"; this is
replaced later by the permanent dentition. In lower vertebrates
the teeth are replaced as worn away. There is not, however,
so great an antithesis in this matter between the Mammalia